As Google-parent Alphabet prepares to report third quarter earnings on Thursday, some Wall Streeters have Amazon on the mind.

While Google and Facebook still have a virtual duopoly on digital advertising revenue, Amazon's business is growing faster than expected, cementing it as a third, increasingly competitive player, according to eMarketer.

"Amazon is becoming a hugely attractive partner next to Google and Facebook in times of increased pressure to prove advertising ROI," Marco Rimini, chief development officer at WPP's Mindshare, told CNBC.

Google's advertising business has continued to post impressive growth, and Wall Street expects that to continue. Its properties revenue, which includes search, has seen year-over-year percentage growth accelerate in the first and second quarter. And analysts estimate that properties revenue will hit $24.3 billion in the third quarter, a more than 20 percent increase year over year, according to FactSet consensus data. Google accounts for a stunning 59 percent of global search ad spending, according to eMarketer.

While Pivotal analyst Brian Wieser also highlighted the Amazon specter in his pre-earnings note, writing that nothing other than Amazon could get in the way of Google's intent-related ad products, the company isn't in any real trouble yet.

As a manager in Google's ad sales organization, speaking on condition of anonymity, recently put it: "Leadership is definitely concerned, but [it's] not a huge threat right now."

While Amazon is a distant threat to Google in search ads, Google is its own distant threat to Amazon in the cloud.

Wall Street will be closely watching the growth of Google's "other" revenues category, which includes Google's cloud business, and will be looking for hints about the business during the company's earnings call. Last quarter, the "other" category, which also includes hardware and sales of apps and content through the Google Play Store, hit $4.4 billion, or a 36.5 percent increase year over year.

Amazon, which is still the leader in the space, will also report its earnings on Thursday.